The present invention relates to an improved racket frame, and more particularly relates to improvements in an FRP racket frame having a face formed by a latticework of strings held in tension on a substantially oval head.
Recent development of racket frames covers a wide variety of types from wooden through metallic to FRP racket frames. Concurrently, a wide variety of materials have been used for strings forming the face of a racket frame. In the case of an FRP racket frame, a foam resin core is wholly embraced by an FRP shell.
The most conventional FRP racket frames have a common, general construction in which a head coupled to a grip via a yoke has a substantially oval shape defining a face formed by a latticework of strings held in tension on the head. The head includes a pair of opposite longitudinal center zones located astride the longer axis of the oval, a pair of lateral center zones located astride the shorter axis of the oval, and four intermediate zones interspacing adjacent center zones. The head also has inner and outer peripheral surfaces substantially parallel to each other, both perpendicularly intersecting the plane of the face. The head is provided with the first group of through, string holes each of which extends parallel to the longer axis of the oval and opens in the inner and outer peripheral surfaces of the head. The head is further provided with the second group of through, string holes each of which extends parallel to the shorter axis of the oval and opens in the inner and outer peripheral surfaces of the head. Each string is held in tension on the head in an arrangement such that, in a circumferential groove formed in the outer peripheral surface of the head, the string runs in the circumferential direction astride the section of the outer peripheral surface between a pair of adjacent string hole of a same group. A string protector is inserted into each string hole for protection of the associated string from vibrations caused by striking balls. This string protector is comprised of a flange section resting in the circumferential groove in the outer peripheral surface and a tubular section inserted into the string hole and projecting somewhat from the inner peripheral surface of the head.
When a string vibrates in a direction perpendicular to the face of the racket frame due to striking balls, its knot of vibration is located at the inner end of the associated string hole or at the inner end of the associated string protector and the vibrating string impinges against such a knot of vibration. As a consequence, the string span is limited by the dimension of the inner peripheral surface of the head or by the dimension of the contour defined by the inner ends of the string protectors. This restricts enlargement in effective surface area of the face of the racket frame, in particular free adjustment in sweet spot, thereby seriously deteriorating feel at striking. Such impingement further makes the position of the knot of vibration quite ambiguous.